[Fleet] Fleet preconfigured variable value is not used

[nchaulet]

Description

Looks like in #112702 we introduced a bug that cause preconfigured package policy to use default value instead of the one provided in the config.

This seems to cause issue for all the variables with default value.

How to reproduce

If you try to add the following preconfigured policy to your kibana config

xpack.fleet.agentPolicies:
  # Cloud Agent policy
  - name: Elastic Cloud agent policy test
    description: Default agent policy for agents hosted on Elastic Cloud
    is_default: false
    is_managed: true
    id: test-1234
    namespace: default
    monitoring_enabled: []
    package_policies:
      - name: fleet_server_test_123
        package:
          name: fleet_server
        inputs:
          - type: fleet-server
            keep_enabled: true
            vars:
              - name: host
                value: 127.0.0.1
                frozen: true

This result in an invalid value in the preconfigured package policy

@joshdover @jen-huang @kpollich Should|Could we still backport to 7.16.0 a fix for that bug? I think the bug is currently mitigated in 7.16.0 by the fact that the cloud policy use the same value as the default value for variables with default value.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)